The present invention relates to a manufacturing method for an active-matrix liquid crystal display apparatus (hereinafter, referred to as "TFT-LCD") wherein a TFT (thin film transistor) is used as a switching element. More specifically, the present invention concerns a method for manufacturing a TFT array substrate which is less susceptible to point defects and line defects, and its objective is to improve the display characteristics and productivity of a TFT-LCD.
A TFT-LCD in which an active-matrix substrate, a color filter substrate also serving as an opposing electrode and liquid crystal are assembled on a glass substrate has been put into the market as a flat display that meets demands for a flat image-display apparatuses, and has been expected to develop a big market as a display for notebook personal computers as well as for office automation monitors.
In a TFT, in most cases, amorphous silicon which can be deposited on a large area at comparatively low temperatures is used as a semiconductor layer.
For example, FIG. 4 shows a cross-sectional view of an essential portion of a TFT array substrate obtained by a conventional manufacturing method.
A first conductive thin film is formed on a glass substrate 51. Next, the first conductive thin film is patterned in a first photolithography process so that a gate electrode 52 and an auxiliary capacitive electrode are formed. Then, a gate insulating film 53, an a-Si:H (amorphous silicon to which hydrogen atoms are added) semiconductor layer 54, an n+a-Si:H ohmic contact layer 55 are deposited by the plasma CVD method.
Next, the semiconductor layer and the ohmic contact layer are patterned in a second photolithography process so that portions to form channels are left. Then, a second conductive thin film is formed. Moreover, the second conductive thin film is patterned in a third photolithography process so that a pixel electrode 56 is formed. Next, contact holes used for forming terminal sections are patterned in a fourth photolithography process. Then, a third conductive thin film such as an Al--Si alloy is formed. Next, the third conductive thin film is patterned in a fifth photolithography process so that a source electrode 57 and a drain electrode 58 are formed, and using the source electrode 57 and the drain electrode 58 as masks, the ohmic contact layer 55 is patterned so as to form a thin film transistor.
Then, a passivation film 59 is formed by a method such as the plasma CVD method, and contact holes used to connect TCPs are patterned in a sixth photolithography process, thereby completing the formation of a thin film transistor array substrate.
Upon forming the gate electrode 52 and the auxiliary capacitive electrode during the process for manufacturing the TFT, a material such as Cr, which has high chemical resistance and a high fusing point, is often used as the material for the gate electrode 52. In order to pattern Cr, a wet etching process is carried out by using an etchant containing cerium ammonium nitrate.
After the gate electrode 52 has been patterned, a gate insulating film 53 is formed, or a gate insulating film 53, an a-Si:H (amorphous silicon to which hydrogen atoms are added) semiconductor layer 54, and an n+a-Si:H ohmic contact layer 55 are continuously formed by the plasma CVD method, and the semiconductor layer 54 and the ohmic contact layer 55 are shaped into an island pattern. Then, a pixel electrode 56 is formed, and after contact holes used for forming contact sections have been formed, a conductive thin film, such as an Al--Si film, is formed so as to form a source electrode 57 and a drain electrode 58. At this time, since the adhesive strength between the glass substrate 51 and the gate insulating film 53 is weak, peeling off of the film tends to occur and cause disconnection of the source wiring, etc., during post processes, resulting in degradation in the display.
Here, Japanese Unexamined Patent Publication No. 203982/1993 discloses a method in which, in order to form a thin film transistor having high reliability, after washing a glass substrate by a hydrofluoric acid solution, an insulating film is formed thereon; however, in order to prevent film rising due to the insufficient adhesive strength, after washing a glass substrate by a hydrofluoric acid solution in the same manner, a gate electrode is formed, and in the case when, after formation of the gate electrode, a gate insulating film is formed without carrying out washing by a hydrofluoric solution, it is not possible to solve the above-mentioned problem.
Moreover, Japanese Unexamined Patent Publication No. 217919/1989 discloses a method in which the adhesive strength between the glass substrate and the gate insulating film is increased in the case when, after a gate electrode has been patterned by dry etching using fluorine-type gas, a gate insulating film is formed; however, the problem with this method is that, in the case of a high concentration of the hydrofluoric acid, or in the case of a long processing time, the surface of the glass substrate becomes so rough that the adhesive strength is lowered conversely.